U.S. patent application number 12/722073 was filed with the patent office on 2011-06-02 for cvd reactor with energy efficient thermal-radiation shield.
This patent application is currently assigned to Woongjin polysilicon Co., Ltd.. Invention is credited to Jong Rock KIM, Jong Gyu LEE, Sang Woo LEE, Lyle C. WINTERTON.
Application Number | 20110126761 12/722073 |
Document ID | / |
Family ID | 43035010 |
Filed Date | 2011-06-02 |
United States Patent
Application |
20110126761 |
Kind Code |
A1 |
LEE; Jong Gyu ; et
al. |
June 2, 2011 |
CVD REACTOR WITH ENERGY EFFICIENT THERMAL-RADIATION SHIELD
Abstract
A Siemens type CVD reactor device is provided. One or more
radiation shields are disposed between a rod filament and a cooled
wall in the reactor. The radiation shield absorbs radiant heat
emanating from the heated polysilicon rod during the CVD process,
gets heated above 400.degree. C., re-radiate the absorbed heat
toward both of the polysilicon rod and the cooled wall, so as to
provide thermal shielding effect to the cooled wall. The net energy
loss of the polysilicon rod is reduced as much as the amount of
energy emitted toward the polysilicon rod from the radiation
shield, such that considerable amount of electrical energy of the
CVD reactor is reduced and saved. The energy reduction rate goes up
much higher if using multiple layered radiation shields, low
shielding emissivity, and low thermal conductivity together. The
purity of the manufactured polysilicon can be maintained by using
thermal shielding material that is stable in a high temperature
such as graphite, silicon carbide-coated graphite, and silicon.
Inventors: |
LEE; Jong Gyu; (Daejeon,
KR) ; KIM; Jong Rock; (Chungcheongbuk-do, KR)
; LEE; Sang Woo; (Daegu, KR) ; WINTERTON; Lyle
C.; (Gyeongsangbuk-do, KR) |
Assignee: |
Woongjin polysilicon Co.,
Ltd.
Gyeongsangbuk-do
KR
|
Family ID: |
43035010 |
Appl. No.: |
12/722073 |
Filed: |
March 11, 2010 |
Current U.S.
Class: |
118/719 ;
118/724 |
Current CPC
Class: |
C23C 16/46 20130101;
C23C 16/4418 20130101; C23C 16/24 20130101; C01B 33/035
20130101 |
Class at
Publication: |
118/719 ;
118/724 |
International
Class: |
C23C 16/00 20060101
C23C016/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 2, 2009 |
KR |
10-2009-0118543 |
Claims
1. A chemical vapor deposition (CVD) reactor device comprising: a
reaction container comprising one or more reaction chambers with a
cooled wall; a plurality of electrodes extending into the reaction
chambers; at least one rod filament having two ends connected to
two different electrodes of the plurality of electrodes in the
reaction chamber and being heated to a high temperature when an
electrical current passes through the two electrodes; a
silicon-containing gas source that is connected to inside of the
reaction chamber, supplies silicon-containing gas into the reaction
chamber, and has polysilicon deposited on surfaces of the rod
filament heated by a chemical vapor deposition (CVD) process
producing polysilicon rod; and a radiation shield disposed between
the rod filament and the cooled wall and/or between the rod
filament and the floor of the reaction chamber and shielding the
radiant heat energy from the polysilicon rod from transferring to
the cooled wall and/or to the floor of the reaction chamber.
2. The CVD reactor of claim 1, wherein the temperature of the
radiation shield is maintained above 400.degree. C. during a CVD
reaction.
3. The CVD reactor of claim 1, wherein the radiation shield is made
to have thickness and thermal conductivity satisfying a condition
of k/.tau. value below 3,000 Watt/Kelvin, where k is a thermal
conductivity of the radiation shield and .tau. is a thickness of
the radiation shield.
4. The CVD reactor of claim 1, wherein the silicon-containing gas
is a silane gas source selected from the group consisting of
monosilane, disilane, or chlorosilane, or mixture thereof.
5. The CVD reactor of claim 1, wherein the radiation shield is made
of any one or combination of two or more selected from the group
consisting of silicon, graphite, silicon carbide (SiC), silicon
carbide-coated material, silicon nitrides (nitrified silicons),
silicon oxides, aluminum oxides, boron nitrides, molybdenum or
molybdenum-based alloys, tungsten or tungsten-based alloys,
tantalum or tantalum-based alloys, silica-based porous materials,
aluminosilicate-based porous materials, gold-coated porous
materials, gold-coated materials, platinum-coated porous materials,
platinum-coated materials, silica-coated porous materials,
silica-coated materials, silver-coated porous materials,
silver-coated materials, and perlite.
6. The CVD reactor of claim 1, wherein the radiation shield is
installed so as to enclose the rod filament and to cover at least a
part of surface of the cooled wall with respect to the cooled wall,
and so as to cover at least a part of the floor with respect to the
floor of the reaction chamber.
7. The CVD reactor of claim 1, wherein the radiation shield
comprises a plurality of radiation shields, and wherein the
plurality of radiation shields are disposed with structures of,
viewing from the rod filament toward the cooled wall, a) being
disposed with two or more layers overlapped, b) being disposed with
single layers with intervals therebetween, or c) being disposed
with mixed structures of the overlapped multiple layers and the
separated single layers.
8. The CVD reactor of claim 1, wherein the radiation shield is
formed by laminating a plurality of radiation shields, and wherein
the plurality of radiation shields are overlapped loosely such that
there exist multiple gaps between layers.
9. The CVD reactor of claim 1, wherein the temperature of a surface
facing the heated polysilicon rod of the radiation shield is
maintained above 400.degree. C. during a CVD reaction.
10. The CVD reactor of claim 1, wherein the radiation shields are
disposed with intervals such that a minimal minute gap is provided
against the cooled wall so as to function as a thermal
resistance.
11. The CVD reactor of claim 1, wherein the radiation shields touch
a surface of the cooled wall loosely such that a plurality of gaps
are provided against the cooled wall.
12. The CVD reactor of claim 1, wherein the radiation shield is
made of material having a thermal conductivity below 35 W/m-k.
13. The CVD reactor of claim 3, wherein the radiation shield is
installed pressed closely to the cooled wall such that there is no
gap resistance to the thermal conductivity of the cooled wall.
14. The CVD reactor of claim 1, wherein the radiation shield
reduces the thermal energy loss from the polysilicon rod by at
least one effect out of a thermal shielding effect by a hot
re-radiation (higher than about 400.degree. C.), a shielding effect
by multiple layers of radiation shields, a shielding effect by a
low spectral emissivity of the radiation shield material, and an
insulating effect by reduction of thermal conductivity due to
thickness thereof.
15. The CVD reactor of claim 1, wherein the radiation shield is
made of material having a surface spectral emissivity from about
0.05 to about 1.0.
16. A chemical vapor deposition (CVD) reactor device comprising: a
reaction container forming a reaction chamber with a base plate and
cooled wall covering the base plate; an electrical power supply
extending from outside of the reaction container into the reaction
chamber through the base plate, being provided with a plurality of
electrodes at end portions; at least one rod filament having two
ends connected to two different electrodes of the plurality of
electrodes of the electrical power supply in the reaction chamber
so as to form a closed circuit and being heated to a high
temperature when an electrical current passes therein through the
electrical power supply; a silicon-containing gas source that
supplies silicon-containing gas into the reaction chamber through a
gas input pipe and a gas output pipe connected from outside to
inside of the reaction chamber, and has polysilicon deposited on
surfaces of the rod filament heated by a chemical vapor deposition
(CVD) process producing polysilicon rod; and a radiation shield
disposed between the rod filament and the cooled wall and/or
between the rod filament and the floor of the reaction chamber,
covering at least a part of the cooled wall and/or a floor of the
reaction chamber, and maintaining a temperature thereof above about
400.degree. C. by absorbing heat radiating from the polysilicon rod
during the CVD reaction and reducing thermal energy loss of the
polysilicon rod by re-radiating a part of the absorbed heat toward
the polysilicon rod.
17. The CVD reactor of claim 16, wherein the radiation shield has
thickness and thermal conductivity satisfying a condition of
k/.tau. value below 3,000 Watt/Kelvin, where k is a thermal
conductivity of the radiation shield and .tau. is a thickness of
the radiation shield.
18. The CVD reactor of claim 16, wherein the radiation shield is
made of any one or combination of two or more selected from the
group consisting of silicon, graphite, silicon carbide (SiC),
silicon carbide-coated material, silicon nitrides (nitrified
silicons), silicon oxides, aluminum oxides, boron nitrides,
molybdenum or molybdenum-based alloys, tungsten or tungsten-based
alloys, tantalum or tantalum-based alloys, silica-based porous
materials, aluminosilicate-based porous materials, gold-coated
porous materials, gold-coated materials, platinum-coated porous
materials, platinum-coated materials, silica-coated porous
materials, silica-coated materials, silver-coated porous materials,
silver-coated materials, and perlite.
19. The CVD reactor of claim 16, wherein the radiation shield
comprises a plurality of radiation shields, and wherein the
plurality of radiation shields are disposed with structures of,
viewing from the rod filament toward the cooled wall, a) being
disposed with two or more layers overlapped, b) being disposed with
single layers with intervals therebetween, or c) being disposed
with mixed structures of the overlapped multiple layers and the
separated single layers.
20. The CVD reactor of claim 16, wherein the radiation shield is
formed by laminating a plurality of radiation shields, and wherein
the plurality of radiation shields are overlapped loosely such that
there exist multiple gaps between layers.
21. The CVD reactor of claim 16, wherein the radiation shields are
disposed with intervals such that a minimal minute gap (empty
space) is provided against the cooled wall so as to function as a
thermal resistance.
22. The CVD reactor of claim 16, wherein the radiation shields
touch a surface of the cooled wall loosely such that a plurality of
gaps is provided against the cooled wall.
23. The CVD reactor of claim 16, wherein the radiation shield is
made of material having a thermal conductivity below about 35
W/m-k.
24. The CVD reactor of claim 17, wherein the radiation shield is
installed pressed closely to the cooled wall such that there is no
gap resistance to the thermal conductivity of the cooled wall.
25. The CVD reactor of claim 16, wherein the radiation shield
reduces the thermal energy loss from the polysilicon rod by at
least one effect out of a thermal shielding effect by a hot
re-radiation (higher than about 400.degree. C.), a shielding effect
by multiple layers of radiation shields, a shielding effect by a
low spectral emissivity of the radiation shield material, and an
insulating effect by reduction of thermal conductivity due to
thickness thereof.
26. The CVD reactor of claim 16, wherein the radiation shield is
made of material having a surface spectral emissivity from about
0.05 to about 1.0.
Description
TECHNICAL FIELD
[0001] The invention relates to an improvement of a CVD reactor and
more specifically to an art to improve the productivity of
polysilicon rods as well as to lower the costs for producing them
by reducing electrical energy loss in Siemens CVD reactors.
BACKGROUND ART
[0002] Polycrystalline silicon, or polysilicon, is a critical raw
material for the electronics industry. It is the starting material
for production of single and multi-crystal silicon ingots for the
semiconductor and photovoltaic industries. Semiconductor grade
polysilicon contains electronically-active impurities in the parts
per billion or parts per trillion ranges.
[0003] Generally, polysilicon rods are made by the pyrolytic
decomposition of a gaseous silicon compound, such as mono-silane or
a chlorosilane (e.g., trichlorosilane) on a rod-shaped, red-heated
starter rod or filament made preferably from a silicon seed rod or,
alternatively, from a high-melting point metal having good
electrical conductivity such as tungsten or tantalum. The
principles of the design of present state-of-the-art reactors for
the pyrolysis of monosilane and chlorosilanes are set forth in, for
example, U.S. Pat. Nos. 3,011,877; 3,147,141; 3,152,933 which are
incorporated herein by reference as if set out in full. Reactors of
this type are commonly referred to as Siemens reactors.
[0004] FIG. 1 shows a structure of a conventional Siemens CVD
reactor. In general, a conventional Siemens CVD reactor device (10)
is a processing container in which a bell-shaped reactor (or a bell
jar) (30) is fixed on a base plate (40) with a gas-tight flange
(33), and one or more reaction chambers (25) are provided inside.
The bell-shaped reactor (30) comprises an outer jacket (30b) and an
inner shell (30a), and since it is configured that coolant flows in
between (a coolant input pipe (31a) and a coolant output pipe (31b)
are connected to the outer jacket (30b)), the reactor's inner shell
(30a) (referred as `cooled wall` hereafter) is maintained at a
state of a temperature lower than 200.degree. C. (100.degree. C. in
a certain process) in the CVD process. The base plate (40) is
connected to an input pipe (48a) and an output pipe (48b) for
flowing coolant, so as to limit the temperature below a
predetermined value. The base plate (40) also comprises a gas inlet
(42) and a gas outlet (44). Silicon-containing gas compounds flow
into the reaction chamber (25) through the gas inlet (42) connected
to a silicon-containing gas source (46), and the gas after CVD
reaction is discarded outside the reaction chamber (25) through the
gas outlet (44). Also, two feedthroughs (38) extend from the
outside of the base plate (40) into the reaction chamber (25), and
each end portion of them is connected to an electrode (39) made of
graphite, for example, while supported by a rod support (37). In
the reaction chamber (25), more than one set of rod filaments (34)
are provided. Specifically, one set of rod filaments (34) forms a
hair-pin or U-shaped rod with two vertical rod filaments (34a, 34b)
standing apart with an interval in the reaction chamber (25) and a
horizontal rod filament (34c) connecting top end portions of the
two vertical rod filaments (34a, 34b). And, each of bottom end
portions of the two vertical rod filaments (34a, 34b) is connected
to an external electrical power supply through an electrode (39)
and the feedthrough (38), and thus the one set of rod filament (34)
forms a complete electrical circuit.
[0005] In this conventional Siemens CVD reactor device (10), a
current flows in the rod filament (34) through the feedthrough (38)
and the electrode (39) for a CVD process, and silicon-containing
gas compounds like monosilane, disilane, or chlorosilane, or
mixture of such gases are supplied to the reaction chamber (25)
from the silicon-containing gas source (46). Then, the rod filament
(34) is heated and a pyrolysis of monosilane to form silicon and
hydrogen as a by-product, or a chlorosilane which produces silicon
and chloride-containing compounds such as HCl, SiHCl.sub.4 or the
like as well as hydrogen, is performed in a Siemens reactor (25). A
mechanism for CVD deposition by the pyrolysis process may be
represented with the following reaction formulas.
SiH.sub.4-->Si+2H.sub.2
SiHCl.sub.3+H.sub.2<-->Si+3HCl
SiHCl.sub.3+HCl<-->SiCl.sub.4+H.sub.2
[0006] As shown in the above, the polysilicon is produced by
chemical vapor deposition (CVD) after heterogeneous decomposition
of the monosilane or chlorosilane onto the glowing hot silicon rod
filament (34). The diameter of a depositing polysilicon rod (36)
increases until the desired size is achieved at which point the
reactor device (10) is shut down, purged of its process gases from
the reaction chamber (25), and the reactor (30) is opened for
harvesting the polysilicon rod (32).
[0007] Currently only the Siemens process has the capability to
produce semiconductor-grade polysilicon. But the Siemens process is
also known as an energy intensive process and so the cost to
produce semiconductor grade silicon using the Siemens process is
relatively high. The electrical energy-cost to produce polysilicon
in a Siemens reactor typically runs between 65 and 200 kWh/kg
resulting in electrical energy being the single largest cost
factor. Therefore, reducing the electrical energy-cost in producing
polysilicon is closely related to competitivity of a polysilicon
manufacturer. Energy from the red-heated rod (32) is lost via
thermal-radiation directly to the surfaces of the cooled wall
(30a), via convection to the process gases in the reaction chamber
(25) and by conduction to the cooled rod supports (37) or
electrodes (39). Especially, the thermal radiant losses directly to
the cooled wall (30a) typically comprise 75% to 95% of the total
energy losses. Therefore, it is required to find a way to reduce
the thermal radiant energy loss effectively.
DISCLOSURE
Technical Problem
[0008] There exists a mechanism for regulating thermal radiant
energy referred to as `cold reflection`. Thermal radiation is
defined as radiant energy emitted by a medium by virtue of its
temperature. The emission of thermal radiation is governed by the
temperature of the emitting body (i.e. heated silicon rod (32)).
When radiant energy impacts a surface medium, this energy can be
either reflected by the medium, absorbed by the medium, or
transmitted through the medium. If the incident medium is highly
polished, radiation is primarily reflected. In pure reflection, the
impacted medium does not absorb radiation, and therefore the medium
temperature does not change. This is a "cold reflector". In the
case of this "cold reflector" the reflected radiation is not
thermal radiant energy.
[0009] U.S. Pat. No. 4,173,944 (Silver-plated vapor deposition
chamber) to Koppl et al. teaches an example of a mechanism for
reducing energy using such a cold reflector. Koppl et al. disclose
a method of plating silver on the inner surface of the cooled wall
(30a), that is, a surface of the inner shell (30a) that lies close
to the silicon rod (32). Plating the inner surface of the cooled
wall (30a) with reflective silver may be for reducing energy by
increasing the wall reflectivity and reflecting the radiant energy
from the polysilicon rod (32) in order to raise the efficiency of
electrical energy. The silver-plated surface is formed as a part of
the cooled wall (30a) which is relatively cooler yet some of the
radiant energy incident on the silver-plated surface is constantly
lost through the cooled wall (30a), and thus the silver-plated
surface is maintained as a relatively cold surface (a little higher
than the temperature of the cooled wall (30a)) all the time. Still,
the silver-plated surface absorbs some radiant energy. But the
silver-plated surface is unable to re-radiate the incident thermal
radiation because it is relatively cold and contributes to energy
efficiency merely by reflecting the thermal radiation incident on
it. But, maintaining the reflectivity of the silver-plated surface
of the cooled wall (30a) in a Siemens reactor as high as its
initial state is difficult because of the harsh process environment
and the process deposits precipitated onto the silver-plated cooled
wall (30).
[0010] It is another burden to maintain the silver-plated surface
of the cooled wall (30a) as a bright glossy surface in order to
have a high reflectivity. In spite of such efforts, the lowering of
the reflectivity of the silver-plated surface is unavoidable, and
the reduction of electrical energy is abated. Also, the cooled wall
(30a) is usually kept below a temperature of about 100.degree. C.
(or 200.degree. C.), and if the temperature goes up above about
250.degree. C. (or 400.degree. C.) then impurities from the cooled
wall (30a) may contaminate the reaction chamber (25). Because of
this, the cooled wall (30a) must be kept below 250.degree. C. (or
400.degree. C.)
[0011] There had been other attempts for raising the energy
efficiency in manufacturing polysilicon using Siemens CVD reactors.
Some companies recover waste heat from the wall coolant in the
cooled wall (30a) of the CVD reactor. But, such waste heat recovery
requires relatively high coolant temperatures which in turn require
hot reactor walls. But, because metals outgas impurities at high
temperatures, hot metal walls are undesirable. Hot emitters could
be designed by using internal insulation, but historically
companies that produce polysilicon using the Siemens process
hesitated to add anything to the interior of the reaction chamber
(25) of the Siemens CVD reactor due to contamination concerns.
[0012] Another problem associated with high radiant energy loss is
a steep radial temperature gradient from the surface to the center
of the heated silicon rod. As energy is lost at the rod surface,
the radial gradient through the center of the rod is increased to
maintain the silicon rod surface at a temperature appropriate for
process kinetics. In U.S. Pat. No. 6,221,155, Keck teaches that the
amperage within electrically heated rods migrates toward the
centers of the silicon rods because of the inverse temperature vs.
resistivity behavior of silicon. Also, Keck teaches that the
silicon rod skin acts as an insulator to energy generated within
the rod core. This migration of amperage and the insulating effect
of the rod skin cause the rod center to run hotter than the skin
resulting in high tensile stresses at the center of the silicon rod
upon cool down, which stresses result in brittle fracture when the
stresses exceed the ultimate strength of the material. Further,
Keck teaches that because rod internal stresses increase with
increasing diameter large diameter silicon rods (greater than
150-mm) cannot be reliably grown using the Siemens CVD process
without reducing the temperature gradient through the silicon
rod.
[0013] The melting point of polysilicon is about 1414.degree. C. If
the silicon rod skin temperature is at the optimum temperature for
the decomposition of trichlorosilane (about 1150.degree. C.) and
the center of the polysilicon rod is elevated in temperature,
occasionally when the polysilicon rod diameter becomes large, the
center of the rod will reach melting point causing rod integrity
failure. To avoid melting, often the skin surface temperature
should be reduced when the silicon rod diameter is large. This
reduction in skin temperature causes a loss in
kinetic-rate-of-decomposition and a reduction in reactor
capacity.
[0014] It is necessary to consider and solve the above problems for
reducing energy in manufacturing polysilicon rod using the Siemens
reactors. From these viewpoints, it is an object of the present
invention to provide a CVD reactor device which can not only raise
the energy efficiency by reducing electrical energy for
manufacturing polysilicon rods, but also maintain the capability of
energy reduction constantly even for extended operation.
[0015] Also, it is another object of the present invention to
provide a CVD reactor device, which can reduce temperature gradient
within the entire inside of the polysilicon rod by reducing net
energy loss of the polysilicon rod during the CVD process and
produce a polysilicon rod of large diameter by reducing tensile
stresses.
[0016] Furthermore, it is still another object of the present
invention to provide a CVD reactor device, which can reduce
electrical energy consumption through processes that do not produce
impurities contaminating the processing environment in the reaction
chamber.
Technical Solution
[0017] To achieve the above objects, the inventors took hot
emission into consideration as a mechanism to adjust the radiant
energy. When radiant energy is absorbed by a non-reflecting medium,
the medium becomes hot. As the temperature of this perfect radiator
or "black-body" medium increases, it begins to emit thermal
radiation according to the Stefan-Boltzmann law which is expressed
as equation (1). Such medium is called "hot emitter".
q=.sigma.AT.sup.4 (1)
where [0018] q=Thermal radiation emitted from emitting body [0019]
.sigma.=Stefan-Boltzmann constant [0020] A=Emitting body surface
area [0021] T=Absolute temperature of the emitting body.
[0022] Applicant checked through various means that the hot emitter
mechanism is an efficient means for solving the problems, and
succeeded in providing a CVD reactor that solves the above
problems. According to an aspect of the present invention, a CVD
reactor device comprises: a reaction container comprising one or
more reaction chambers with a cooled wall; a plurality of
electrodes extending into the reaction chambers; at least one rod
filament having two ends connected to two different electrodes of
the plurality of electrodes in the reaction chamber and being
heated to a high temperature when a current flow through the two
electrodes; a silicon-containing gas source that is connected to
inside of the reaction chamber, supplies silicon-containing gas
into the reaction chamber, and has polysilicon deposited on
surfaces of the rod filament heated by a chemical vapor deposition
process producing polysilicon rod; and a radiation shield disposed
between the rod filament and the cooled wall and/or between the rod
filament and the floor of the reaction chamber and shielding the
radiant heat energy from the polysilicon rod from transferring to
the cooled wall and/or to the floor of the reaction chamber.
[0023] The "hot emitter" thermal radiation shield (or shields),
which is placed between the heated polysilicon rod and the cooled
wall surfaces, intercepts the thermal radiation emanating from the
polysilicon rods casting a shadow onto the cooled wall. As the
gray-body shield absorbs the thermal radiant energy emanating from
the polysilicon rod in the CVD process, its temperature increases.
The "hot emitter" shield emits increasing quantities of thermal
radiation as its temperature increases, some of the radiant heat
toward the heated polysilicon rods and the rest of the radiant heat
toward the cooled wall. The portion of the thermal radiation energy
emitted toward the polysilicon rods reduces the rods net energy
loss. The radiation shield is preferably kept at a temperature
above 400.degree. C. during the CVD process. Especially, the
temperature of a surface facing the heated polysilicon rod of the
radiation shield is preferably kept above 400.degree. C.
[0024] According to another aspect of the present invention, a CVD
reactor device comprises: a reaction container forming a reaction
chamber with a base plate and a cooled wall covering the base
plate; an electrical power supply extending from outside of the
reaction container into the reaction chamber through the base
plate, being provided with a plurality of electrodes at end
portions; at least one rod filament having two ends connected to
two different electrodes of the plurality of electrodes of the
electrical power supply in the reaction chamber so as to form a
closed circuit and being heated to a high temperature when a
current flow therein through the electrical power supply; a
silicon-containing gas source that supplies silicon-containing gas
into the reaction chamber through a gas input pipe and a gas output
pipe connected from outside to inside of the reaction chamber, and
has polysilicon deposited on surfaces of the rod filament heated by
a chemical vapor deposition process to produce polysilicon rod; and
a radiation shield disposed between the rod filament and the cooled
wall and/or between the rod filament and the floor of the reaction
chamber, covering at least a part of at least one of the cooled
wall and the floor of the reaction chamber, and maintaining a
temperature thereof above about 400.degree. C. by absorbing heat
radiating from the polysilicon rod during the CVD reaction and
reducing thermal energy loss of the polysilicon rod by re-radiating
a part of the absorbed heat toward the polysilicon rod.
[0025] In the CVD reactor device, it is preferable that the
radiation shield encloses the rod filament, and is installed so as
to cover at least a part of surface of the cooled wall with respect
to the cooled wall and so as to cover at least a part of the floor
with respect to the floor (plate) of the reaction chamber.
[0026] The core of the polysilicon rod is elevated in temperature
above that of the skin. Reducing energy loss of the polysilicon rod
reduces the rod's radial thermal gradient and also the resulting
tensile stresses at the rod core, allowing one to successfully grow
polysilicon rods to a larger diameter. The production capacity of
the Siemens CVD reactor increases as the polysilicon rod diameter
increases.
[0027] It is more preferable that a plurality of hot emitter
thermal radiation shields are installed in multiple layers.
Multiple "hot emitter" thermal radiation shields (or multiple
layers within a single shield) retain energy more efficiently than
one single shield with only one layer. Each layer of the radiation
shields contributes independently to rod-energy retention. When
stacked in multiple layers, the plurality of radiation shields are
preferably disposed loosely such that multiple gaps are provided
between the multiple layers.
[0028] In installing the plurality of radiation shields, it is
preferable that the plurality of radiation shields are disposed
with structures of, viewing from the rod filament toward the cooled
wall, a) being disposed with a plurality of layers having more than
two layers overlapped, b) being disposed with single layers with
intervals therebetween, and c) being disposed with mixed structures
of the overlapped multiple layers and the separated single
layers.
[0029] The hot emitter radiation shields are preferably made of
material with low thermal conductivity. The radiation shields of
considerable thickness and low thermal conductivity behave like
thermal insulators by restricting heat transfer through the
thickness of the radiation shield thereby causing the shield
surface facing the thermal radiation source (that is, the heated
polysilicon rod) to emit thermal radiation and thereby add to the
overall energy conservation.
[0030] The radiation shield is preferably made to maintain its
temperature above 400.degree. C. during the CVD reaction.
Especially, the temperature of a surface of the radiation shield
facing the heated polysilicon rod is preferably kept above
400.degree. C. during the CVD reaction.
[0031] The radiation shields are preferably disposed with intervals
such that a minimal minute gap is provided against the cooled wall
so as to function as a thermal resistance. However, the radiation
shield may be installed to contact loosely the surface of the
cooled wall such that a plurality of gaps (empty space) against the
cooled wall are provided.
[0032] The radiation shield is preferably made to have thickness
and thermal conductivity satisfying a condition of k/.tau. value
below 3,000 watt/Kelvin. Here k is a thermal conductivity of the
radiation shield, and .tau. is a thickness of the radiation shield.
If satisfying the condition, the radiation shield may be installed
closely to the cooled wall such that there is no gap resistance to
the thermal conductivity with the cooled wall.
[0033] The radiation shield is preferably made of material having a
spectral emissivity of 0.05-1.0.
[0034] The radiation shield is preferably made of silicon,
graphite, or silicon carbide (SiC). The material for the radiation
shield is not limited to the above though. The radiation shield can
be made of any one or combination of two or more selected from the
group consisting of, including the above-mentioned materials,
silicon carbide-coated material, silicon nitrides (nitrified
silicons), silicon oxides, aluminum oxides, boron nitrides,
molybdenum or molybdenum-based alloys, tungsten or tungsten-based
alloys, tantalum or tantalum-based alloys, silica-based porous
materials, aluminosilicate-based porous materials, gold-coated
porous materials, gold-coated materials, platinum-coated porous
materials, platinum-coated materials, silica-coated porous
materials, silica-coated materials, silver-coated porous materials,
silver-coated materials, and perlite.
[0035] The radiation shield reduces the thermal energy loss from
the polysilicon rod by at least one of the following effects;
thermal shielding effect by a hot emittance (>400.degree. C.),
shielding effect by multiple layers of radiation shields, shielding
effect by a low emissivity of the radiation shield material, and
insulating effect by reduction of thermal conductivity due to
thickness. The efficiency of the shield is strongly dependent on
its physical position within the reaction chamber. The shield must
be placed in such a way that its source-facing surface (surface
facing the heated polysilicon rod) is maintained hot (high
temperature) by intercepting the thermal radiant energy leaving the
heated rods.
Advantageous Effects
[0036] If radiation shields are installed between the polysilicon
rod filament and the cooled wall in the Siemens CVD reactor as
suggested by the present invention, the thermal loss from surface
of the polysilicon in the reactor is reduced by blocking the
thermal energy radiating from the heated filament rod toward the
cooled wall. As a result, the net energy loss of the polysilicon
rod is reduced, and the total electrical energy consumption of the
CVD reactor can be reduced by the temperature control mechanism of
the Siemens CVD reactor.
[0037] Additionally, by reducing the energy loss of the polysilicon
rod, the temperature difference between the core and the surface of
the polysilicon rod is reduced and the resulting tensile stress is
reduced. Therefore, it is enabled to grow a polysilicon rod to a
large diameter, increasing the productivity of the Siemens CVD
reactor.
DESCRIPTION OF DRAWINGS
[0038] FIG. 1 is an axial view showing structure of a conventional
Siemens CVD reactor;
[0039] FIG. 2 is an axial view showing structure of a CVD reactor
with a radiation shield installed so as to cover entire surface of
a cooled bell jar wall of the CVD reactor according to an
embodiment of the present invention;
[0040] FIG. 3 is a cross-sectional view showing along line A-A of
FIG. 2;
[0041] FIG. 4 is a cross-sectional view showing structure of a CVD
reactor with a radiation shield installed so as to cover a part of
the surface of a cooled wall of the CVD reactor according to
another embodiment of the present invention;
[0042] FIG. 5 is an axial view showing structure of a CVD reactor
with a multiple layered thermal radiation shield installed so as to
cover a part of the surface of a cooled wall of the CVD reactor
according to still another embodiment of the present invention;
[0043] FIG. 6 is a cross-sectional view showing along line B-B of
FIG. 5;
[0044] FIG. 7 is a cross-sectional view showing structure of a CVD
reactor with a radiation shield having 3 separated-layers installed
so as to cover an entire or a part of inner surface of the reactor
according to still another embodiment of the present invention;
[0045] FIG. 8 is a cross-sectional view showing a radiation shield
having 3 loosely-overlapped-layers applied loosely to a surface of
a cooled wall;
[0046] FIG. 9 is a graph illustrating a relationship between
temperature of inner surface of a radiation shield and energy
reduction rate;
[0047] FIG. 10 is a graph illustrating a relationship between
features of a radiation shield and energy reduction rate;
[0048] FIG. 11 is an axial view showing structure of a CVD reactor
with a radiation shield installed so as to cover a cooled wall and
a floor of the reactor according to still another embodiment of the
present invention; and
[0049] FIG. 12 is a plan view showing along line C-C of FIG.
11.
BEST MODE
[0050] FIG. 2 is an axial view showing structure of a Siemens CVD
reactor device (100) according to a first embodiment of the present
invention. The CVD reactor device (100) further comprises a hot
emitter radiation shield (or radiation shields) compared to a
conventional CVD reactor device (10), and the structures other than
a thermal radiation shield (20) are same. This hot emitter
radiation shield (20) is installed between a polysilicon rod (32)
(that is, rod filament (34)) and a cooled wall (30a) of the
bell-shaped reactor (30). FIG. 3 is a cross-sectional view along
the line A-A of FIG. 2. As illustrated, it is preferable that the
radiation shield (20) encloses the rod filament (34) and covers the
entire inner surface of the cooled wall (30a). However, the
radiation shield (20) may be installed so as to cover just a part,
not the whole, of the cooled wall (30a). For example, FIG. 4 shows
that radiation shields (120) having four parts are installed so as
to cover partially side portions of each quadrant, a first quadrant
to a fourth quadrant (viewing from a center of the reaction chamber
(25)). Also, as shown in FIGS. 5 and 6 the radiation shield (220)
can be installed so as to cover partially a side portion of the
cooled wall (30a) but not to cover a top portion of the cooled wall
(30a) (For example, the radiation shield (220) shown in FIGS. 5 and
6 comprises multiple layers, which is represented by solid lines).
In other structures, the radiation shield (220) can be installed so
as to cover the entire side portions, but not the top portion of
the cooled wall (30a). In addition, the radiation shield can be
installed variously so as to cover a part of the cooled wall (30a).
Irrespective of the structure as in the above, if the radiation
shield is installed to cover at least a part of the cooled wall
(30a), a radiation heat shielding effect corresponding to the
covered area can be provided.
[0051] A "hot-emitter" thermal radiation shield (or shields) (20)
intercepts the thermal radiation emanating from the polysilicon
rods (32) and casts a shadow onto the cooled wall (30a) to provide
heat shielding effect. The gray-body radiation shield (20)
intercepts and absorbs the thermal radiant energy emanating from
the polysilicon rod (32) causing a temperature in the reactor to
rise. The "hot emitter" thermal radiation shield (20) emits
increasing quantities of thermal radiation as its temperature
increases, some toward the heated polysilicon rods (32) and some
toward the cooled wall (30a). At some temperature below that of the
heated polysilicon rods (32) and above that of the cooled wall
(30a), the shield reaches an equilibrium or steady state
temperature.
[0052] The net energy loss between the heated polysilicon rod (32)
and the cooled wall (30a) is modeled with the well known
Stefan-Boltzmann law. For two concentric cylinders of equal surface
areas the net energy transfer becomes:
q=.sigma.A(T.sub.1.sup.4-T.sub.2.sup.4)/(1/.epsilon..sub.1+(1/.epsilon..-
sub.2-1)) (2)
[0053] where [0054] q=Thermal radiation emitted [0055]
.sigma.=Stefan-Boltzmann constant [0056] A=Surface area of the rod
(32) [0057] T.sub.1=Absolute temperature of the rod (32) [0058]
T.sub.2=Absolute temperature of the cooled wall (30a) [0059]
.epsilon..sub.1=Spectral emissivity of the rod (32) [0060]
.epsilon..sub.2=Spectral emissivity of the cooled wall (30a).
[0061] As can be seen from Equation (2), because the net energy
loss between the polysilicon rod (32) and the cooled wall (30a) is
directly proportional to the 4th power temperature difference
between them and because the temperature difference between the
polysilicon rod (32) and the radiation shield (20) is lower than
the temperature difference between the polysilicon rod (32) and the
cooled wall (30a), the net energy loss from the heated polysilicon
rod (32) is reduced by the addition of a "hot emitter" thermal
radiation shield (20). As the thermal radiation emitted from the
polysilicon rod (32) surface decreases (assuming constant energy
input to the polysilicon rod (32)) the surface temperature of the
polysilicon rod (32) increases. But because the Siemens CVD process
requires a specific surface temperature of the polysilicon rod
(32), a temperature-control-loop of the process acts to decrease
the electrical energy input to the polysilicon rod (32) to maintain
a constant surface temperature. Hence energy is conserved.
[0062] For a Siemens CVD reactor, the hot emitter thermal radiation
shield (20) is most useful if its temperature is maintained at or
above 400.degree. C. Applying real numbers to the Stefan-Boltzmann
law generates the graph of FIG. 9, which gives % energy savings as
a function of innermost surface temperature of the radiation shield
of Siemens CVD reactor device (100). According to the graph, the
radiation shield (20) of the Siemens CVD reactor device (100) at
400.degree. C. reduces the thermal radiation energy loss from the
heated polysilicon rods (32) by about 3.9%. The energy reduction
must be by at least 4% for an `effective` reduction effect.
Therefore, in reducing the energy of the CVD reactor device (100),
the inner temperature of the radiation shield (20) must be
maintained at or above 400.degree. C. in order for the energy
reduction to be effective. As illustrated in the graph, the energy
reduction rate increases as the inner temperature of the radiation
shield increases, such that the energy reduction rate is about 10%
for an inner temperature of the radiation shield of about
550.degree. C., and rises to about 64% for an inner temperature of
1000.degree. C. Thus, it is preferable that the radiation shield
(20) is formed and installed such that its inner surface
temperature is maintained to be high.
[0063] A preferable way to maintain the inner surface temperature
of a hot emitter thermal radiation shield (20) at a required high
temperature is to provide a gap between the radiation shield (20)
and the cooled wall (30a). The thermal radiant energy leaving the
heated polysilicon rod (32) is absorbed by the thermal radiation
shield (20) and causes the temperature of the radiation shield (20)
to increase. If incoming energy flow to the radiation shield (20)
is fixed, the final steady-state temperature of the thermal
radiation shield (20) depends upon the rate at which its energy is
lost. A gap between the thermal radiation shield (20) and the
cooled wall (30a) stops energy loss of the radiation shield (20) by
direct conduction to the cooled wall (30a). The gap distance is not
important in blocking thermal conduction to the cooled wall. For
example, a very minute gap such as 10 .mu.m can be meaningful.
[0064] The required hot surface of a hot-emitter thermal radiation
shield (20) can be created by providing a shield that restricts
conduction to the cooled wall (30a). If the radiation shield (20)
is tightly pressed up against the cooled wall (30a) such that there
is no gap resistance to conduction heat transfer between the
thermal radiation shield (20) and the cooled wall (30a), a
combination of thickness and low thermal conductivity of the
radiation shield (20) can also restrict that energy loss from the
thermal radiation shield (20) such that a temperature of the
surface facing the heated polysilicon rod (32) is maintained above
400.degree. C. Energy transfer through the thermal radiation shield
(20) is represented by Fourier's Law of Conduction,
q=k/.tau.A.DELTA.T (3)
[0065] where [0066] q=Energy transferred [0067] k=thermal
conductivity of material of the radiation shield (20) [0068]
A=Cross sectional area of heat transfer [0069] .DELTA.T=Temperature
difference (front to back of radiation shield (20)) [0070]
.tau.=Thickness of thermal radiation shield (20).
[0071] The term k/.tau. (that is, ratio of thermal conductivity (k)
of material of the radiation shield to thickness (.tau.) of the
radiation shield) must be at or below about 3,000 Watt/Kelvin in
order to effectively restrict heat transfer to the cooled wall
(30a) such that the thermal radiation shield (20) temperature is
maintained at or above 400.degree. C.
[0072] As the energy generated within the heated polysilicon rod
(32) due to resistive heating decreases (due to the presence of a
thermal radiation shield (20)) and as the radiant energy losses at
the rod (32) surface decrease, the radial temperature gradient in
the polysilicon rod (32) also decreases. Assuming no conduction
losses at the rod supports (39, 37, and 38), the heat-energy
generated within the polysilicon rod (32) due to resistive heating
must all be dissipated at the surface of the polysilicon rod (32)
by thermal-radiation and convection. Fourier's Law of Conduction
shows clearly that a decrease in energy transfer causes a decrease
in radial temperature-gradient (.DELTA.T/.DELTA.r) which reduction
results in lower residual rod stresses after the polysilicon rods
reach desired size and are cooled to room temperature.
[0073] Lower stresses allow increased diameters of polysilicon rod
(32) and a corresponding increase in the production capacity of the
Siemens reactor device (100). A production capacity of the Siemens
reactor device (100) is limited by the available surface area of
the rod filaments (34) or polysilicon rods (32) present within the
process chamber (25). If one grows polysilicon rods (32) to a
larger diameter, the average available surface area increases
because rod surface area is linear with diameter. More area allows
silicon bearing gas flow to increase into the process chamber (25)
which in turn will be deposited in a form of polysilicon on the rod
filaments (34) and finally result in a corresponding increase in
average production capacity of the Siemens CVD reactor (100).
[0074] Also, a decreased thermal-gradient of the polysilicon rod
(32) allows one at large diameters to operate the rod surface at a
fixed optimum temperature of 1150.degree. C. (for the CVD
decomposition of trichlorosilane, for example) without concerns
that the center of the polysilicon rod (32) will reach melting
point causing an early batch termination.
[0075] Multiple radiation shields (or multiple layers within a
single shield) retain energy more efficiently than one single
shield. FIG. 7 shows that three layers of radiation shields (320-1,
320-2 and 320-3) form a set of radiation shield (320) and covers
sides of the cooled wall (30a). Each radiation shield (320-1,
320-2, 320-3) works as an additional layer of energy transfer
resistance by absorbing and re-emitting energy back to the source.
Each radiation shield (320-1, 320-2, 320-3) has two surfaces, the
first surface oriented toward the radiant source (that is, toward
the polysilicon rod (32)) and the opposite surface oriented toward
the cooler target, the cooled wall (30a). Let's suppose that two
radiation shields (320-1, 320-3) are placed between the polysilicon
rod (32) and the cooled wall (30a) and the radiation shield-1
(320-1) is closest to the polysilicon rod (32) and radiation
shield-3 (320-3) is closest to the cooled wall (30a). In this case,
the radiation shield-3 (320-3) intercepts thermal radiant energy
emanating from the radiation shield-1 (320-1). The temperature of
radiation shield-3 (320-3) increases as it absorbs energy. As the
temperature of radiation shield-3 (320-3) increases, the net energy
transfer from the radiation shield-1 (320-1) to the radiation
shield-3 (320-3) decreases according to the Stefan-Boltzmann law
because the 4th power temperature difference between the radiation
shields (320-1, 320-3) decreases. As in the above example each
layer of the multiple shielding contributes independently to
radiant energy interception, and thus the effect of multiple
shielding increases as high as each layer decreases the total
energy transfer. Therefore, it is preferable to use as many layers
of thermal radiation shielding as possible.
[0076] If two radiation shields are tightly pressed against each
other such that there is almost no gap (empty space) therebetween,
the heat transfer between the radiation shields occurs via
conduction (not thermal radiation) and they behave as if they were
one shield. However, if two radiation shields are "loosely
touching" each other such that numerous gaps exist between the
layers, each layer behaves as a fraction of a shield or a partial
shield. The radiation shield (420) shown in FIG. 8 is formed such
that the layers of three radiation shields (420-1, 420-2, 420-3)
are contacted at a certain locations (contact points) directly and
a lot of gaps (empty space) are provided between the layers. Like
this, it is convenient to use a multi-layer thermal-radiation
shield (420) which is constructed by loosely laying multiple layers
of shielding material together.
[0077] Also, the radiation shield may be installed so as to contact
the cooled wall (30a). FIG. 8 shows that the radiation shield
(420), more specifically the outermost layer of radiation shield
(420-3), is contacted very loosely to a surface of the cooled wall
(30a). In such a case, the radiation shield (420) loses energy by
thermal conduction through the contacted portion to the cooled wall
(30a), whereas the gap (empty space) between the portion not
contacted to the cooled wall (30a) and the cooled wall (30a) allows
heat transfer by radiation only, but blocks heat transfer by
conduction. For example, when the thermal radiation shield (420-3)
is occasionally touching the cooled wall (30a) such that surfaces
touching comprise only 5% of the total shield surface area, the
thermal energy is lost by conduction to the cooled wall (30a) but
conduction heat transfer from the thermal radiation shield (420-3)
to cooled wall (30a) would be restricted by about 95%.
[0078] A combination of the effects described above can also be
effective at maintaining the thermal radiation shield at or above
400.degree. C. For example, if the thermal radiation shield is
touching the cooled wall (30a) but multiple gaps remain between the
thermal radiation shield and the cooled wall (30a), energy losses
via conduction will be limited. As shown in FIG. 8, if the three
layers of radiation shield (420-1, 420-2, and 420-3) make a set of
radiation shield (420) by a "loosely touching" or "lightly pressed
touching" and the radiation shield (420) provides a lot of gaps
against the cooled wall (30a), it can provide a good thermal
shielding effect.
[0079] With a proper combination of substantial thickness and low
thermal conductivity of a radiation shield such that k/.tau., the
ratio of the substantial thickness to the thermal conductivity, is
near or below 3,000 Watt/Kelvin, a loosely touching radiation
shield can maintain itself at or above 400.degree. C. For example,
one may construct a loosely packed 100-layer shield that behaves as
though it were made of 50 individual shields with full gaps between
shields.
[0080] A thin radiation shield with material k/.tau.>3,000
Watt/Kelvin with multi-layers (or multiple-overlapped) pressed
tightly against the cooled wall (30a), such that no gaps exist
between the shield layers and between the radiation shield and the
cooled wall (30a), behaves like no radiation shield exists because
the radiation shield surface temperature is substantially same as
that of the cooled wall (30a). Therefore for energy savings such a
thermal-radiation shield should be either located in the gas space
between the polysilicon rods (32) and the cooled wall (30a) (that
is, installed so as not to touch the surface of the cooled wall
(30a) directly) or it should be installed "loosely" attached to the
cooled wall (30a) such that many gaps exist between the radiation
shield and the cooled wall (30a), or it should be constructed with
thickness and thermal conductivity such that k/.tau.<3,000
Watt/Kelvin (in this case, the radiation shield may be installed to
be pressed to the cooled wall (30a)), or it should be a thermal
radiation shield that uses a combination of these design
parameters.
[0081] If the shield material has considerable thickness and low
thermal conductivity such that k/.tau.<3,000 Watt/Kelvin, it may
behave like a thermal insulator by restricting heat transfer
through the thickness of the radiation shield. The overall effect
is that the total thermal-radiant energy transferred is reduced. In
such a case the incident surface of the radiation shield operates
at a substantially higher temperature than the opposite
surface.
[0082] The Stefan Boltzmann law shows that the spectral surface
emissivity of the radiation shield affects the energy transfer.
"Hot Emitter" thermal radiation shield materials with surface
emissivities covering the whole range of 0.05 to 1.0 are efficient
in inhibiting energy transfer. Those with lowest spectral surface
emissivity are the most efficient because reflection occurs in one
direction only while emittance occurs in two directions (both
surfaces of the radiation shield).
[0083] On the other hand, critical for installing a radiation
shield in a Siemens CVD reactor is that the radiation shield
material be non-contaminating. At elevated temperature most
materials will react, pyrolyze or outgas impurities. Graphite will
slowly react with hydrogen at elevated temperatures forming methane
gas and the addition of silicon-carbide as a coating over graphite
comprises a hydrogen-impervious layer that eliminates or reduces
the reaction between carbon and hydrogen. Hence, silicon carbide
coated graphite is well suited as a thermal radiation shield
material. Silicon carbide, silicon carbide coating over graphite,
silicon, silicon coating over graphite, similar high purity
materials, or their coatings over generic high temperature
materials could be suitable for use as radiation shield materials
in a Siemens CVD reactor. Those skilled in the art could choose
from a wide variety of materials. For example, the radiation shield
can be made of any one or combination of two or more selected from
the group consisting of silicon nitrides (nitrified silicons),
silicon oxides, aluminum oxides, boron nitrides, molybdenum or
molybdenum-based alloys, tungsten or tungsten-based alloys,
tantalum or tantalum-based alloys, silica-based porous materials,
aluminosilicate-based porous materials, gold-coated porous
materials, gold-coated any materials, platinum-coated porous
materials, platinum-coated any materials, silica-coated porous
materials, silica-coated any materials, silver-coated porous
materials, silver-coated any materials, and perlite.
MODE FOR INVENTION
[0084] To demonstrate shield performance of the radiation shield,
three examples are given and compared to a base-case, that is, to a
conventional Siemens CVD reactor device shown in FIG. 1 without a
radiation shield. The base-case consumes 133 kW of electrical
energy continuously at 115 mm rod diameter and the maximum rod-core
temperature is 1233.degree. C., for the conventional Siemens CVD
reactor.
Embodiment 1
[0085] A SiC-coated graphite thermal-radiation shield was placed
between the heated rod filaments and the cooled wall of a Siemens
CVD reactor. The 5 mm thick radiation shield covered the entire
surface of the cooled wall but did not cover the surface of the
base plate. The radiation shield was positioned with a 100 mm gap
from the cooled wall. The spectral emissivity of the SiC coated
graphite was considered to be 1.0. For a 115-mm diameter of
polysilicon rod the total power consumption in the CVD reactor was
101 kW resulting in 24% electrical energy savings compared to the
base-case. The polysilicon rod-core temperature was 1194.degree. C.
corresponding to a rod-core temperature decrease of 39.degree. C.
from the base-case.
Embodiment 2
[0086] A perlite thermal-radiation shield was placed between the
rod filaments and the cooled wall of a Siemens CVD reactor. The 100
mm thick radiation shield covered the entire surface of bell
jar-shaped cooled wall, but did not cover the surface of the base
plate. The radiation shield was positioned with a 5 mm gap from the
cooled wall. The spectral emissivity of the perlite was considered
to be 0.9 and the perlite thermal conductivity was 0.029 W/mK. For
a 115 mm diameter of polysilicon rod, the total power consumption
of the CVD reactor was 86 kW resulting in 35% electrical energy
savings compared to the base-case. The polysilicon rod-core
temperature was 1178.degree. C. corresponding to a rod-core
temperature decrease of 54.degree. C. from the base-case.
Embodiment 3
[0087] Three silicon thermal-radiation shields were placed between
the rod filaments and the cooled wall of a Siemens CVD reactor. The
silicon shields were configured as shown in FIG. 4. The 5 mm thick
(ea) radiation shields covered the entire bell jar-shaped cooled
wall, but did not cover the surface of the base plate. The
radiation shields were positioned with a 5 mm gap from the cooled
wall. The spectral emissivity of the silicon was considered to be
0.9. For a 115 mm diameter of polysilicon rod, the total power
consumption of the CVD reactor was 52 kW resulting in 61%
electrical energy savings compared to the base case. The
polysilicon rod-core temperature was 1145.degree. C. corresponding
to a rod-core temperature decrease of 88.degree. C. from the
base-case.
[0088] The heat shield effects mentioned above, such as heat shield
by virtue of high temperature emittance (>400.degree. C.), heat
shield by multiple layers of radiation shield, heat shield by low
emissivity spectral emittance of shield material and heat shield by
thermal insulation due to a lowering of conduction through the
thickness of the shield, are all additive regarding reducing the
overall energy loss from the polysilicon rod (32). In order to
check these unquantified effects in detail, calculations were
performed to observe how much the spectral surface emittance,
shield thickness, shield thermal conductivity, and number of layers
of a radiation shield material affect the electrical energy
reduction. The results of the calculations are the graph shown in
FIG. 10. When the radiation shield is formed with 3 layers, the
energy reduction rate is improved by about 6.4% compared to a case
of just 1 layer. When the spectral surface emittance of the
radiation shield material is increased from 0.9 to 1.0, the energy
reduction rate is improved by about 1%. When the thickness of the
radiation shield is increased from 1 mm to 10 mm, the energy
reduction rate is improved by almost 2%. Also, when the thermal
conductivity of the radiation shield material is changed from 0.025
W/m-k to 35 W/m-k, the energy reduction rate is increased by almost
17%. According to the calculations, for a radiation shield having a
thickness of 1 mm and a thermal conductivity of 35 W/m-k, the
energy reduction rate is below 4%. Therefore, it is preferable to
make a radiation shield with material having a thermal conductivity
below 35 W/m-k.
[0089] In the above embodiments a radiation shield is installed
between a polysilicon rod (32) and a cooled wall (30a), but the
radiation shield can be installed on a floor of a reaction chamber
(25), that is, above a base plate (40). Since the thermal energy
loss in the reaction chamber (25) can be reduced through the
radiation shield covering the base plate (40), the net energy loss
of the polysilicon rod (32) can be reduced much further. In the
result, the energy reduction rate can be further raised, and the
radial thermal gradient of the polysilicon rod (32) can be lowered.
FIGS. 11 and 12 show cases in which a radiation shield is installed
not only between the polysilicon rod (32) and the cooled wall
(30a), but also between the floor of the reaction chamber (25),
that is, top of the base plate (40), and the polysilicon rod (32).
As illustrated, the two-layered radiation shields (420-1, 420-2)
are installed between the polysilicon rod (32) and the cooled wall
(30a) covering the entire cooled wall (30a) except a part of the
cooled wall (30a) (for observing inside of the reaction chamber
(25) through a window (not shown) from outside), and as for the top
side of the base plate (40) radiation shields (420-3, 420-4) can be
installed so as to cover almost all area except for electrodes
(39), rod support (37), gas inlet (42), and gas outlet (44). In
order to minimize heat loss through conduction, the radiation
shields (420-3, 420-4) are preferably installed so as not to touch
directly the base plate (40), the electrodes (39) and the rod
support (37) disposed on the base plate (40), and the gas inlet
(42) and gas outlet (44).
INDUSTRIAL APPLICABILITY
[0090] The present invention can be used to reduce electrical
energy reduction of a Siemens CVD reactor, and at the same time to
lower a temperature of center of a polysilicon rod.
* * * * *